It's like picking the known evil vs the unknown. And, I know it's probably not a good answer logically but more gut driven.

Emails have a lot of information about my life.

I know what Google is doing with my emails, and as far as I can see Google is not going to die or be purchased by another entity.

With companies like Protonmail and others I just don't know. Even if I trust their current T&C, what happens if they are bought.

Also, while it is equally likely for Google to have rogue employees, I believe they will have more stringent safeguards then smaller companies.

Again, as I said, probably not a very good answer, but more gut driven. And, I say this as someone who is very conscious of privacy.